The XRD pattern demonstrates that the products grown under hydrothermal conditions are SnO2 of good crystallinity, with the obtained diffraction peaks, broadened by the small diameter of
Trang 1Synthesis of one-dimensional SnO 2 nanorods via a hydrothermal technique
O Lupana,b, , L Chowa, G Chaic, H Heinricha,d,e, S Parka, A Schultea
a
Department of Physics, University of Central Florida, PO Box 162385, Orlando, FL 32816-2385, USA
b
Department of Microelectronics and Semiconductor Devices, Technical University of Moldova, 168 Stefan cel Mare Blvd., Chisinau MD-2004, Republic of Moldova
c Apollo Technologies, Inc 205 Waymont Court, S111, Lake Mary, FL 32746, USA
d
Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA
e
Department of Mechanical, Materials, Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA
a r t i c l e i n f o
Article history:
Received 19 August 2008
Received in revised form
7 October 2008
Accepted 8 October 2008
Available online 17 October 2008
PACS:
81.10.Dn
61.46.w
61.46.Km
68.37.Lp
78.30.Fs
Keywords:
SnO 2 nanorod
Crystal structure
Semiconductors
Hydrothermal synthesis
Raman spectra
a b s t r a c t
We have developed a simple solution process to synthesize tin oxide nanorods The influence of precursors and the reaction temperature on the morphology of SnO2is investigated SnO2nanorods are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Raman spectroscopy The as-grown SnO2nanorods are uniform in size with a radius of 50–100 nm and length of 1–2mm The nanorods grow direction is parallel to the [1 0 1] direction Possible growth mechanism of SnO2nanorods is discussed
&2008 Elsevier B.V All rights reserved
1 Introduction
Controlled synthesis of nanostructures is an important step for
the manufacturing of nanodevices Performance of semiconductor
nanodevices may depend on their morphology Recently,
one-dimensional (1D) materials have attracted great interest due to
their potential applications as interconnects and functional
components[1–5] 1D oxide nanostructures showed interesting
properties, chemical and thermal stability, diverse functionalities,
high durability, owing to their high degree of crystallinity[3],and
emerge as nanoscale building blocks for electronic and
optoelec-tronic devices[4,5] At the same time, the interest in developing
parts per billion (ppb)-level gas sensors requires new approaches
and new nanomaterials One of the most important sensor
materials is tin oxide (SnO2), which is a low-cost, large-bandgap
(3.6 eV, at 300 K), and n-type semiconductor[6] SnO2’s properties
are greatly affected by the size and morphology, which define
their further applications Thus, designing SnO21D nanorods and nanoarchitectures with well-defined morphologies is of impor-tance for fundamental research and high-tech applications Fabrication of SnO2nanorods has been accomplished using several vapor deposition techniques, such as rapid oxidation[7], chemical vapor deposition (CVD)[8], and thermal evaporation[9] Peng et al
[10] have recently reported a hydrothermal synthesis of SnO2
nanorods However, organic reagents such as hexanol and sodium dodecylsulfate used in the synthesis of SnO2nanorods can lead to undesirable impact on human health and on the environment [6] Zhang et al [11] also reported a low-temperature fabrication (at
200 1C for 18 h) via a hydrothermal process of crystalline SnO2
nanorods Vayssieres et al.[12]reported SnO2nanorods arrays grown
on F-SnO2glass substrates by aqueous thermohydrolysis at 95 1C
In this work we report a simple, one-step low-temperature aqueous synthesis of SnO2 1D nanorods without the need of templates or surfactants
2 Experimental details The SnO2 nanorods were synthesized via a hydrothermal method, which is similar to the method used in SnO2microcubes
Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/physe
Physica E
1386-9477/$ - see front matter & 2008 Elsevier B.V All rights reserved.
Corresponding author at: Department of Physics, University of Central Florida,
4000 Central Florida Blvd PO Box 162385, Orlando, FL 32816-2385, USA.
Tel.: +1 407 823 5217.
E-mail address: lupan@physics.ucf.edu (O Lupan).
Trang 2[13]and ZnO nanorods synthesis[14] In a typical synthesis, 50 ml
SnCl4aqueous solution (in deionized (DI) water resistivity—18.2
MOcm) in the presence of 1 ml of HCl (37%) and NH4OH (29.5%)
(Purchased from Fisher Scientific) solution was mixed and stirred
for 5 min The mixing solution was then transferred to a reactor
[14] It was heated to 95 1C and kept for 15 min Then the system
was allowed to cool to 40 1C naturally A silicon substrate were
cleaned as previously described[15]and placed inside the reactor
The structural properties of SnO2nanorods were determined
by X-ray diffraction (XRD) (Rigaku ‘D/Max-b(R)’ X-ray
diffract-ometer with CuKa radiation and a normaly–2yscan) [14] The
morphologies of the SnO2 nanorods were studied by a scanning
electron microscopy (SEM) Transmission electron microscopy
(TEM) observation of the samples was performed with a FEI
Tecnai F30 TEM operated at an accelerating voltage of 300 kV For
the TEM observation, the products were collected on a holey
carbon grid Micro-Raman measurements were performed at
room temperature on a Horiba Jobin Yvon LabRam IR system at a spatial resolution of 2mm Raman scattering was excited with the
633 nm line of a He–Ne laser with output powero4 mW at the sample
3 Results and discussion
Fig 1 shows the SEM images of the as-grown products synthesized on SiO2/Si substrates The products consist of nanorods as well as nanoparticles The diameters of SnO2
nanorods are in the range of 100–150 nm with lengths of the order of 1–2mm The end planes of the nanorods are tetragonal (see insetFig 1a)
The morphology of nanorods is found to be dependent on the synthesis conditions The dimensions and aspect ratio are a function of growth time, temperature and Sn+/OH ratio in solution
The XRD pattern of SnO2nanorods is shown inFig 2 There are peaks with 2y values of 26.971, 34.341, 38.261, 52.011, 54.901, 71.281, and 78.401, corresponding to SnO2tetragonal rutile crystal planes of (11 0), (1 0 1), (2 0 0), (2 11), (2 2 0), (2 0 2), and (3 2 1), respectively Observed peaks can be indexed to the rutile-structured SnO2 with lattice constants a ¼ b ¼ 0.4738 nm and
c ¼ 0.3185 nm, (JCPDS-PDF# 021-1250)(ICSD data)[16] Two SiO2
peaks were observed at 44.701, a reflection from (11 4), and at 68.871, a reflection from (7 8 3) planes The XRD pattern demonstrates that the products grown under hydrothermal conditions are SnO2 of good crystallinity, with the obtained diffraction peaks, broadened by the small diameter of the nanorods
A TEM image of a single SnO2 nanorod with a diameter of about 100 nm is shown inFig 3 Further characterization was performed using HRTEM Fig 3b shows an HRTEM image of a nanorod The corresponding selected-area electron diffraction (SAED) pattern taken from a section of the nanorod shown inFig
3c can be indexed to the tetragonal cell with lattice constants of
a ¼ 0.474 nm and c ¼ 0.318 nm, in agreement with the XRD result The SAED pattern also confirms that the nanorod is a single crystalline rutile SnO2 with preferential growth direction along the [1 0 1] direction, confirming the XRD analysis
Raman spectra are sensitive to crystallinity, defects and structural disorder in nanoarchitectures Therefore, the vibra-tional properties of SnO2 nanorods were studied by Raman spectroscopy
Fig 4 shows Raman spectra in Stokes frequency range (200 cm1–850 cm1) of the products annealed for 1 h at 370 1C There are Raman peaks at 475, 632, and 774 cm1in the Raman spectrum which are in agreement with those of a rutile SnO2
Fig 1 SEMs of the SnO 2 nanorods hydrothermally grown (a) and closer view (b).
Inset shows a magnified image of cross section of the SnO 2 nanorods.
Fig 2 The XRD pattern of the SnO 2 nanorods grown via the hydrothermal method.
Fig 3 (a) The TEM image of a SnO 2 nanorod on a holey-carbon TEM grid (b) Enlarged HRTEM image of a single-crystalline SnO 2 nanorod and (c) the corresponding
Trang 3single crystal [17] and in agreement with data of group-theory
analysis[18,19] These peaks are attributed to the Eg, A1g, and B2g
vibrational modes of SnO2[20]
SnO2with the rutile structure belongs to the space group P42/
mnm and point group D4h[21] The k ¼ 0 optical modes and their
infrared (IR) and Raman (R) activity can be presented as follows
[21]:
G¼A1gðRÞ þ A2gðFÞ þ A2uðIR; kÞ þ B1gðRÞ þ B2gðRÞ
þ2B1uðFÞ þ EgðRÞ þ 3EuðIR; ?Þ (1)
Raman active modes are A1g, B1g, B2g, and Eg, in these modes the
oxygen atoms vibrate while the Sn atoms are at rest The Egmode
represents vibrations in the direction of the c-axis, but A1gand B1g
are modes describing vibrations perpendicular to the c-axis[21]
Seven modes of A2uand 3Euare IR active and two modes of A2g
and B1uare inactive[22].Fig 4shows the Raman spectra for A1g
mode which were broadened as the size of SnO2 nanorods
decreased[23] The Eg, A1g, and A2g are depicted in the Raman
scattering spectra and confirm the rutile structure of SnO2
nanorods
4 A proposed growth mechanism
Understanding of the growth mechanism of nanorods without
the need in templates, surfactants or applied field is very
important for the synthesis of new materials as well as for device
applications
A proposed growth mechanism of SnO2 nanorods can be
explained in terms of chemical reactions and crystal growth as
follows From the crystallization point of view, the synthesis of an
oxide during of an aqueous solution reaction is expected to
experience a hydrolysis-condensation (nucleation-growth)
pro-cess
In our experiments, we observe that the shape and aspect ratio
of the as-prepared SnO2 products is changed and decreases by
varying the molar ratio of SnCl4 to NH4OH from 20:1 to 10:1,
which is in agreement with the previous reports[11,24]
Growth of SnO2nanorods occurs according to the total reaction
[11,24]:
The amphoteric hydroxide Sn(OH)4 dissolves in excess of
ammonia solution and forms [Sn(OH)6]2anions:
During the hydrothermal reaction, the [Sn(OH)6]2ions decom-posed into SnO2:
(5)
The appropriate molar ratio of Sn4+to OHions for the growth of SnO2nanorods is found to be 1:25–35 According to SEM images
as presented inFig 2b, decreasing tin ions concentration to 0.02 M and below will produce quasi-spherical particles when all other conditions remain constant By raising the concentration up to 0.015–0.02 M, nanorods will form Change of the tin ion concentration in the reverse micelles is much higher than that
in the bulk solution[25]and thus leads to morphology differences
in agreement with previous reports[11,24] It is also observed that increasing the temperature and extending the heating duration leads to an increase of the surface-to-volume ratio of nanorods The concentration of tin ions in solution also influences to the size
of nanorods
The formation of SnO2 nanorods was obtained with the progress of crystal growth The kinetic growth regime during the hydrothermal reaction (Eq (5)) is a decisive factor in the formation of tetragonal-shaped crystals Here, has to be consid-ered adjusting the concentration of precursors as described above, thus controlling the hydrolysis ratio and quality of the nuclei Also, the hydrothermal temperature is an important factor affecting hydrolysis rate of SnO2 nanorods growth Thus, by regulating these parameters, the nucleation and growth processes directly on substrate can be controlled
In our experiments, the hydrothermal process using described reaction media of aqueous metal–ion precursors allows a slow nucleation and growth at low-interfacial tension conditions, which favors the generation of tetragonal-shaped SnO2 nanocrys-tals In these conditions, the product morphology is dictated by the crystal symmetry as well as by the surface energy in aqueous environment and thus the most stable crystal habit is generated directly onto the substrates, without the need of surfactants or templates[12] Also, the growth mechanism of SnO2nanorods can
be explained on the base of its rutile structure, which is 6:3 coordinated and the bonding between atoms has a strong ionic character The synthesized material is a square cross section nanocrystal (insert in Fig 1) because of the tetragonal unit cell containing two tin atoms and four oxygen atoms As was determined experimentally from our results, the tetragonal crystal growth is enclosed by the stable (11 0) facets, thus the rutile structure is built up from the neutral stacked layers of the following planes (O), (2Sn+O), and (O) with ionic charges 2, 4+, and 2, respectively, in the surface unit cell In this way, it is possible that a termination with these planes of the SnO2(11 0) is called a stoichiometric surface According to the presented results, SnO2 can grow from solutions in well-defined tetragonal edges and giving a proper morphology
Thus, by carefully adjusting of the balance between the thermodynamic and the kinetic growth regime, crystals can be formed with geometrical morphology consistent with its crystal-lographic structure Also controlling the kinetic growth regime can promote the anisotropic growth along the high-energy crystallographic face It is known that SnO2with rutile structure belongs to the (P42/mnm) space group with square pyramid as its thermodynamically stable crystallographic form [12] According
to theoretical studies—the (11 0) surface is the thermodynami-cally most stable termination [26] and has the lowest surface energy It would therefore be expected to be predominantly in the nanomaterials morphology At the same time the surface energy suggest that the (0 0 1) surface is very unstable However, both planes (110 and 001) are close in attachment energies and can be
Fig 4 Micro-Raman scattering spectra of the as-grown SnO 2 nanorods.
Trang 4observed experimentally [12] and demonstrated theoretically
[26,27]
5 Conclusion
SnO2nanorods were successfully grown directly onto SiO2/Si
substrates by a simple hydrothermal method The SnO2nanorods
are grown parallel to the [1 0 1] direction in the tetragonal rutile
structure The microstructures and surface compositions of the
nanorods were characterized by XRD, TEM, SEM, and Raman
spectra
The results from Raman spectra and XRD patterns demonstrate
that the obtained nanorods have the single crystalline rutile
structure of SnO2 The present process has the advantage of being
very simple, its yields are high and the morphology of nanorods
can be controlled
Further work on optimization of the synthetic parameters such
as heating temperature, duration, and rate to control the aspect
ratio and different morphologies of the nanorods is underway in
our laboratory
Acknowledgments
L Chow acknowledges financial support from Apollo
Technol-ogies, Inc and the Florida High Tech Corridor Research Program
The research described here was made possible in part by an
award for young researchers (O.L.) (MTFP-1014B Follow-on) from
the Moldovan Research and Development Association (MRDA),
under funding from the US Civilian Research & Development
Foundation (CRDF) Raman measurements were supported in part
by NSF MRI grant DMR-0421253
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